Four-terminal system for reading the state of a phase qubit

ABSTRACT

Quantum computing systems and methods that use opposite magnetic moment states read the state of a qubit by applying current through the qubit and measuring a Hall effect voltage across the width of the current. For reading, the qubit is grounded to freeze the magnetic moment state, and the applied current is limited to pulses incapable of flipping the magnetic moment. Measurement of the Hall effect voltage can be achieved with an electrode system that is capacitively coupled to the qubit. An insulator or tunnel barrier isolates the electrode system from the qubit during quantum computing. The electrode system can include a pair of electrodes for each qubit. A readout control system uses a voltmeter or other measurement device that connects to the electrode system, a current source, and grounding circuits. For a multi-qubit system, selection logic can select which qubit or qubits are read.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.09/875,776, filed Jun. 5, 2001, entitled “Four-Terminal System ForReading The State Of A Phase Qubit”.

BACKGROUND

1. Field of the Invention

This invention relates to quantum computing and to solid-state devicesthat use superconductive materials to implement the coherent quantumstates used in quantum computing.

2. Description of Related Art

Research on what is now called quantum computing traces back to RichardFeynman. See, e.g., R. P. Feynman, Int. J. Theor. Phys. 21, 467 (1982).He noted that quantum systems are inherently difficult to simulate withclassical (i.e., conventional, non-quantum) computers, but that thistask could be accomplished by observing the evolution of another quantumsystem. In particular, solving a theory for the behavior of a quantumsystem commonly involves solving a differential equation related to thesystem's Hamiltonian. Observing the behavior of the system providesinformation regarding the solutions to the equation.

Further efforts in quantum computing were initially concentrated onbuilding the formal theory or on “software development” or extension toother computational problems. Milestones were the discoveries of theShor and Grover algorithms. See, e.g., P. Shor, SLAM J. of Comput. 26,1484 (1997); L. Grover, Proc. 28th STOC, 212 (ACM Press, New York,1996); and A. Kitaev, LANL preprint quant-ph/9511026. In particular, theShor algorithm permits a quantum computer to factorize large naturalnumbers efficiently. In this application, a quantum computer couldrender obsolete all existing “public-key” encryption schemes. In anotherapplication, quantum computers (or even a smaller-scale device such as aquantum repeater) could enable absolutely safe communication channelswhere a message, in principle, cannot be intercepted without beingdestroyed in the process. See, e.g., H. J. Briegel et al., preprintquant-ph/9803056 and references therein.

Showing that fault-tolerant quantum computation is theoreticallypossible opened the way for attempts at practical realizations. See,e.g., E. Knill, R. Laflamme, and W. Zurek, Science 279, 342 (1998).

Quantum computing generally involves initializing the states of N qubits(quantum bits), creating controlled entanglements among them, allowingthese states to evolve, and reading out the qubits the states evolve. Aqubit is conventionally a system having two degenerate (i.e., of equalenergy) quantum states, with a non-zero probability of being found ineither state. Thus, N qubits can define an initial state that is acombination of 2^(N) classical states. This entangled initial stateundergoes an evolution, governed by the interactions that the qubitshave among themselves and with external influences. This evolution ofthe states of N qubits defines a calculation or in effect, 2^(N)simultaneous classical calculations. Reading out the states of thequbits after evolution is complete determines the results of thecalculations.

Several physical systems have been proposed for the qubits in a quantumcomputer. One system uses molecules having degenerate nuclear-spinstates. See N. Gershenfeld and I. Chuang, “Method and Apparatus forQuantum Information Processing,” U.S. Pat. No. 5,917,322. Nuclearmagnetic resonance (NMR) techniques can read the spin states. Thesesystems have successfully implemented a search algorithm, see, e.g., M.Mosca, R. H. Hansen, and J. A. Jones, “Implementation of a quantumsearch algorithm on a quantum computer,” Nature 393, 344 (1998) andreferences therein, and a number-ordering algorithm, see, e.g., L. M. K.Vandersypen, M. Steffen, G. Breyta, C. S. Yannoni, R. Cleve, and I. L.Chuang, “Experimental realization of order-finding with a quantumcomputer,” preprint quant-ph/0007017 and references therein. (Thenumber-ordering algorithm is related to the quantum Fourier transform,an essential element of both Shor's factoring algorithm and Grover'salgorithm for searching unsorted databases.) However, expanding suchsystems to a commercially useful number of qubits is difficult.

More generally, many of the current proposals will not scale up from afew qubits to the 10²˜10³ qubits needed for most practical calculations.A technology that is excellently suited for large-scale integrationinvolves superconducting phase qubits.

One implementation of a phase qubit involves a micrometer-sized loopwith three (or four) Josephson junctions. See J. E. Mooij, T. P.Orlando, L. Levitov, L. Tian, C. H. van der Wal, and S. Lloyd,“Josephson Persistent-Current Qubit,” Science 285, 1036 (1999) andreferences therein. The energy levels of this system correspond todiffering amounts of magnetic flux threading the loop. Application of astatic magnetic field normal to the loop may bring two of these energylevels (or basis states) into degeneracy. Typically, external ACelectromagnetic fields are also applied, to enable tunneling betweennon-degenerate states.

A radio-frequency superconducting quantum-interference device (rf-SQUID)qubit is another type of phase qubit having a state that can be read byinductively coupling the rf-SQUID to rapid single-flux-quantum (RSFQ)circuitry. See R. C. Rey-de-Castro, M. F. Bocko, A. M. Herr, C. A.Mancini, and M. J. Feldman, “Design of an RSFQ Control Circuit toObserve MQC on an rf-SQUID,” IEEE Trans. Appl. Supercond. 11, 1014(2001) and references therein, which is hereby incorporated by referencein its entirety. A timer controls the readout circuitry and triggers theentire process with a single input pulse, producing an output pulse onlyfor one of the two possible final qubit states. The risk of this readoutmethod lies in the inductive coupling with the environment causingdecoherence or disturbance of the qubit state during quantum evolution.The circuitry attempts to reduce decoherence by isolating the qubit withintermediate inductive loops. Although this may be effective, theoverhead is large, and the method becomes clumsy for large numbers ofqubits.

In both above systems, an additional problem is the use of basis statesthat are not naturally degenerate. Accordingly, the strength of thebiasing field for each qubit has to be precisely controlled to achievethe desired tunneling between its basis states. This is possible for onequbit, but becomes extremely difficult with several qubits.

U.S. patent applications Ser. No. 09/452,749, “Permanent ReadoutSuperconducting Qubit,” filed Dec. 1, 1999, and Ser. No. 09/479,336,“Qubit using a Josephson Junction between s-Wave and d-WaveSuperconductors,” filed Jan. 7, 2000, which are hereby incorporated byreference in their entirety, describe Permanent Readout SuperconductingQubits (PRSQs). An exemplary PRSQ consists of a bulk superconductor, agrain boundary, a superconductive mesoscopic island [i.e., asuperconductive region having a size such that a single excess Cooperpair (pair of electrons) is noticeable], and a means for grounding theisland. The material used in the bulk or the island has asuperconducting order containing a dominant component whose pairingsymmetry has non-zero angular momentum, and a sub-dominant componentwith any pairing symmetry. As a result, the qubit has the states ±Φ₀,where Φ₀ is the minimum-energy phase of the island with respect to thebulk superconductor.

The area in which the phase is maintained is much more localized in aPRSQ than in prior qubits such as an rf-SQUID qubit. Thus, the rate ofdecoherence is minimized, making the PRSQ a strong candidate for futuresolid-state quantum-computing implementations.

The state of a PRSQ can be characterized by the direction of themagnetic field, H↑ or H↓, inside the junction between the bulk and theisland. This difference in field direction can be used to read out thestate of a PRSQ, for instance using a SQUID. However, the proposedreadout methods introduce an interaction with the environment thatpotentially disturbs the state of the qubit, which necessitatescomplicated and time-consuming error-correction and/or re-initializationprocedures. Attempts to “switch off” the readout circuit during quantumevolution face severe practical constraints. For example, physicallymanipulating the distance between the SQUID and qubit prior to readoutcould provide the desired coupling and decoupling but is complex toimplement, while up to the present, no integrated solid-statealternative method is known.

The issues discussed above for rf-SQUID qubits and PRSQs are general.Also, other currently proposed methods for reading out the state of aphase qubit involve detection and manipulation of magnetic fields, whichmake these methods susceptible to decohering noise and limit the overallscalability of the device. Thus, there is a need for an efficientreadout method that is non-destructive and switchable, i.e., that doesnot couple the qubit to the environment during computations.

Our invention invokes the classical Hall effect, which arises from thetangential acceleration of moving charged particles in an externalmagnetic field perpendicular to the velocity of the charged particles.The Hall effect drives oppositely charged particles in oppositedirections and leading to charge build-up on the surfaces. As a result,current flow through a sample in a magnetic field produces a Hallvoltage across the sample, in the direction perpendicular to both thecurrent and the field. The Hall effect can be observed in a sample thatis a conductor or a semiconductor, in which case the charged particlesare the electrons or holes.

The Hall effect can also exist in superconducting structures. For SNS(superconductor-normal conductor-superconductor) junctions, the Halleffect was theoretically described by F. Zhou and B. Spivak in “HallEffect in SN and SNS Junctions,” Phys. Rev. Lett. 80, 3847 (1998); for arelated effect, see A. Furusaki, M. Matsumoto, and M. Sigrist,“Spontaneous Hall effect in chiral p-wave superconductor,” preprintcond-mat/0102143 and references therein. Both articles are herebyincorporated by reference in their entirety.

SUMMARY

In accordance with an aspect of the invention, a phase-qubit device suchas a PRSQ uses a four-terminal readout method and system. One embodimentincludes an insulator over the qubit and conducting electrodes (whichcan be metallic) over the insulator (or two insulators over the qubitwith an electrode over each insulator) to create two extra tunneljunctions across the width of the qubit. A readout process then involvesgrounding the PRSQ island, applying a current bias through the qubit,and measuring the potential drop across the electrodes.

The above embodiment of the invention can take advantage of the Halleffect to measure the magnetic field inside the junction. The currentbias perpendicular to the junction (from bulk to island for example) canbe a pulse. Given the direction of the bias current, the sign of thetime-averaged Hall voltage read out across the width of the junctionindicates the orientation of the magnetic field, i.e., the state of thequbit. In other words, for a given bias current, the expected voltage ispositive or negative depending on the qubit state (H↑ or H↓).

Before reading out the qubit state, the qubit island is grounded for theapplication of the bias current through the qubit. The groundingoperation strongly increases the island capacitance of the island, thus“freezing” the qubit by suppressing tunneling and other quantum effects.Thus, while the grounding connection is closed, the qubit retains thesame state. In an embodiment of the invention, the grounding circuitryincludes a switch such as a single-electron transistor (SET) or paritykey. By modulating the gate voltage on the SET (or the flux through theparity key), the circuit can be opened and closed The SET can operatewith either single electrons or Cooper pairs, depending on theembodiment of the invention.

In one embodiment of the invention, the Josephson junction in the qubitis an SNS structure, in which case the Hall voltage in thenormal-conductor barrier consists of short pulses with a non-zero timeaverage. More generally, readout processes using the Hall effect areapplicable to any type of Josephson junction including, for example,grain-boundary junctions.

In an embodiment of the invention, the resistance of the tunnel barrierthat isolates the qubit from an electrode in the readout system ischosen so as to allow for voltage measurements without introducing alarge, intrusive noise in the qubit. Typically, the tunnel resistance isof the order of 100 kΩ.

Alternative embodiments of the invention can place the electrodessymmetrically or asymmetrically over the junction. Asymmetric placementfurther allows for the readout of a dipole magnetic field inside thejunction, with the Hall electrodes being predominantly sensitive to onedipole component.

The four-terminal readout process does not introduce a strong couplingto the environment and is compatible with the qubit schematics.Additionally, fabrication techniques permit formation of the readoutsystem as part of an integrated structure that can be scaled to includea large number of qubits.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, and 1C illustrate a fabrication process for the readoutsystem for a Permanent Readout Superconducting Qubit (PRSQ).

FIG. 2 is a block diagram illustrating a control system integrated withthe four-terminal readout system.

FIG. 3 illustrates a control system as an integrated circuit for thefour-terminal readout system.

FIG. 4 illustrates a multi-qubit system with a readout control systemand a pair of electrodes for each qubit.

FIG. 5 illustrates a control system as an integrated circuit for amulti-qubit system.

FIG. 6 illustrates a control system as another integrated circuit for amulti-qubit system.

FIG. 7 illustrates a control system as an integrated circuit for amulti-qubit system.

DETAILED DESCRIPTION

In accordance with an aspect of the invention, a system and process readout the state of a qubit including at least one Josephson junction andhaving states corresponding to degenerate ground states of differentmagnetic moments. Such a readout system can include a groundingmechanism, a mechanism for biasing the qubit junction with currentpulses, and a mechanism for determining the potential drop across thewidth of the junction. Given the direction of the bias current, theorientation of the state-dependent magnetic field controls the sign ofthe measured potential drop. In other words, for a given bias current, apositive or negative measured voltage corresponds to the qubit state (H↑or H↓). The readout process can be non-destructive to the qubit state aslong as the applied bias remains below a de-pinning current, whichdepends upon the embodiment of the invention.

For example, if the threshold current of a PRSQ is exceeded, then theflux of the qubit, which defines its state and under normal operation islocalized in the two degenerate ground states, becomes de-pinned, anddynamical effects result. The de-pinning current of the qubit iscorrelated with such aspects as the junction thickness, or the crystalmisalignment between the island and bulk superconductors. Moreover, thede-pinning current can also depend on the qubit state.

In one embodiment, the bias current is less than the de-pinning current,which has a magnitude that is specific to the qubit structure and can bedetermined for each system. In another embodiment of the invention, thebias current can exceed the de-pinning current, thus erasing the qubitstate.

In an embodiment of the invention that applies the four-terminal readoutto a Josephson grain-boundary-junction phase qubit (PRSQ), a mechanismfor measuring the potential drop across the qubit junction can includetwo conductive electrodes separated from each other along the grainboundary and isolated from the qubit by a high-resistance insulator. Thetwo electrodes can be connected to a voltmeter that can detect themagnitude and direction of a potential difference between theelectrodes. The resistance of the insulator depends on the embodiment ofthe invention, but should be chosen to minimize the electrode-qubitcoupling during evolution of the qubit's state.

The electrodes can overlie opposite edges of the Josephson junction witha high-resistance insulator between the Josephson junction and theelectrodes. A device such as a voltmeter, which can be an electrometer,detects the sign of a potential difference between the electrodes duringreadout.

One method for four-terminal readout in accordance with the inventiongrounds the superconducting island so that a bias current can flowthrough the qubit to ground. A bias current is then applied through theJosephson junction by way of connecting a current source in series withthe qubit. The current source can be connected to the bulksuperconductor and ground. The force that the qubit's magnetic field Hexerts on the current through the Josephson junction creates a Hallelectric field across the width of the current flow. The electrodes,which are capacitively coupled through high-resistance barriers to theJosephson junction, develop a voltage difference, reflecting thedirection and magnitude of the magnetic field H.

FIGS. 1A, 1B, and 1C illustrate in plan view a process for fabricating aPRSQ with a four-terminal readout structure in accordance with anembodiment of the invention. FIG. 1A shows a structure 100 including abulk superconductor 10, a mesoscopic superconducting island 20, and ajunction material 15 between bulk 10 and island 20 to form a Josephsonjunction. Fabrication of a structure such as illustrated in FIG. 1A isfurther described in U.S. patent application Ser. No. 09/452,749, whichis incorporated by reference above. The crystal alignments of bulk 10and island 20 can have misorientation angles A₁₀ and A₂₀ respectively,with respect to the junction interface. These misorientation anglesaffect the properties of the qubit associated with the Josephsonjunction. For example, if angle A₁₀ is zero and angle A₂₀ is 45°, thecritical current of the junction will be maximized.

In one embodiment, bulk 10 is a region of superconducting material(e.g., YBCO or YBa₂Cu₃O_(7−x)) that has a pairing symmetry havingnon-zero angular momentum and is formed with a thickness on the order of100 nm on an underlying insulating substrate (not shown). An optionalfinger 12, which is part of bulk superconductor 10, extends towardisland 20. Bulk 10 is large enough to have a fixed phase, with respectto which island 20 has a bi-stable ground-state phase of ±Φ₀. These twoground states make up the basis states of the PRSQ.

Mesoscopic island 20 is a region of superconducting material (e.g.,YBCO) having a dominant pairing symmetry with non-zero angular momentum,formed on the substrate to about the same thickness as bulksuperconductor 10. The dimensions of island 20 are typically on theorder of 1 μm by 1 μm. The width of island 20 controls the width of theJosephson junction and can be less than or equal to the width of finger12.

For a qubit containing an extrinsic Josephson junction, junctionmaterial 15 is typically a normal metal such as gold (Au) and has athickness L₁₅ chosen to optimize the critical current. Alternatively,junction material 15 can be a grain boundary between bulk 10 and island20.

A mechanism 40 for grounding the qubit can include any switch that caneffect a connection between the qubit and ground. Use of a SET forgrounding island 20 was first described in U.S. patent application Ser.No. 09/452,749, incorporated by reference above. The use of SETs is wellknown and operation is well established.

As illustrated in FIG. 1B, the fabrication process adds high-resistanceinsulators 30A and 30B and electrodes 25A and 25B over the junction ofthe qubit. Each insulator 30A or 30B forms a tunnel barrier and can be,for example, a region of a high-temperature superconductor (HTS)material such as YBCO doped or chemically treated to be in theinsulating state. Alternatively, for an extrinsic Josephson junction,such as gold in YBCO/Au/YBCO junction, insulators 30A and 30B can beformed, for example, by depositing, patterning, and oxidizing a thinlayer of aluminum (Al). Each of insulators 30A and 30B has a width W₃₀,which is typically the order of 10 to 20 nm. As an alternative to thetwo separate insulators 30A and 30B illustrated in FIG. 1B, oneinsulator can extend across the entire width of the junction.

Electrodes 25A and 25B, which are on respective insulating region 30Aand 30B, are conducting regions and can be made of a metal such as goldor a semiconductor such as (doped) gallium-arsenide. Electrodes 25A and25B can be formed using conventional deposition and photolithographytechniques.

FIG. 1B shows insulating regions 30A and 30B and electrodes 25A and 25Bpositions relative to junction material 15. Alternatively, regions 30Aand 30B and/or electrodes 25A and 25B can be placed asymmetrically withrespect to the center of the junction, e.g., extending further over bulk10 or further over island 20. Asymmetric placement of electrodes 25A and25B facilitates readout of dipolar magnetic fields in the junction bypositioning electrodes 25A and 25B to be mostly affected by the Halleffect voltage that one pole of the magnetic field causes.

A readout method for a phase qubit such as illustrated in FIG. 1Cincludes: grounding island 20 to allow current flow through the qubit,applying a current through the Josephson junction (from bulk 10 toisland 20 or from island 20 to bulk 10), reading the resulting potentialdifference between terminals 50A and 50B connected to electrodes 25A and25B, and identifying the state of the qubit from the measured voltagedifference. Generally, the magnitude of the voltage difference dependson the particular structure of the qubit, but the sign of the voltagedifference depends solely on the directions of the bias current and thequbit magnetic moment.

FIG. 2 illustrates a quantum computing system including a qubit and acontrol system 80 for reading the state of the magnetic moment of thequbit. Control system 80 generally provides external control of thereadout of the qubit and includes: a mechanism for applying a currentsource or supercurrent source to the Josephson junction, a mechanism forgrounding the qubit, and a voltmeter. The control system interfaces withthe four terminals 74, 76, 78A, and 78B of the readout system. Moreparticularly, control system 80 uses terminal 74 for grounding island20. In an embodiment, terminal 74 connects to the gate of a SET that isbetween island 20 and ground, and during the readout operation, controlsystem 80 controls the gate voltage to cause the SET to conduct.Terminal 76 is connected to bulk superconductor 10, and control system80 temporarily applies a bias voltage or current source to bulk 10 tocause a current through the Josephson junction between bulk 10 andisland 20. Terminals 78A and 78B correspond to terminals 50A and 50B(FIG. 1C) and connect a voltmeter or other measurement device in controlsystem 80 to the electrodes over the Josephson junction for measuring avoltage difference during the readout operation.

An electrode structure 73 of the readout system of FIG. 2 can include aninsulative region and conductive region, wherein the conductive regionis deposited on the insulative region. Electrode system 73 is part ofthe readout system and allows measurement of the potential drop acrossthe width of the qubit.

FIG. 3 illustrates an exemplary embodiment of the circuitry in controlsystem 80. In the illustrated embodiment, control system 80 includesread logic 86, a switch 40, a current source 84, and a voltmeter 82. Fora readout operation, read logic 86 operates switch 40 to directlyconnect island 20 to ground. Switch 40 can be a SET, a parity key, oranother device capable of grounding island 20. Read logic 86 thentemporarily connects current source 84 to bulk superconductor 10 togenerate a current through the junction between bulk superconductor 10and island 20. The magnetic field at the junction creates a voltagedifference across the current flow. Electrodes 25A and 25B, which arealong the Josephson junction between bulk superconductor 10 and island20, are capacitively coupled to the voltage across the current flow, andthe change in the voltage across the current flow generates acorresponding voltage difference between electrodes 25A and 25B.Voltmeter 82, which is connects electrodes 25A and 25B, measures anypotential difference between electrodes 25A and 25B.

In some embodiments of the invention such as illustrated in FIG. 3, readlogic 86 and current source 84 produce a current pulse train with atunable frequency. In such embodiments, voltmeter 82 can be aradio-frequency single-electron transistor (rf-SET), with capability ofdetecting microvolt—picosecond voltages. Operation of rf-SETs is wellknown; see, e.g., R. J. Schoelkopf, P. Wahlgren, A. A. Kozhevnikov, P.Delsing, and D. E. Prober, “The Radio-Frequency Single-ElectronTransistor (rf-SET): A Fast and Ultrasensitive Electrometer,” Science280, 1238 (1998).

An embodiment of a method for reading out the states of multiple qubitscan include readout of one qubit at a time by grounding a selectedqubit, applying a current through the Josephson junction associated withthe selected qubit, and measuring the voltage difference in theelectrode system associated with the selected qubit. The process isrepeated for each qubit where the only difference is modulation of thegrounding switches on each qubit. Reading one qubit at a time, therespective grounding switch is closed, while the other groundingswitches remain open, thus allowing only a single path for the currentto travel along.

FIG. 4 illustrates a control system 80 in a multi-qubit system 400. Inmulti-qubit system 400, a terminal 76 connects to bulk superconductor 10for applying a current source 84 through bulk superconductor 10.Terminals 74-1 to 74-N provide a mechanism for grounding respectiveislands 20-1 to 20-N. Additionally, each qubit 1 to N has a pair ofterminals 78A-1 and 78B-1 to 78A-N and 78B-N for measuring the potentialdrop across readout system 73-1 to 73-N, respectively.

In an embodiment of the invention, selection and read logic selectswhich of the qubits is being read and connects the selected qubit to avoltmeter. Selection logic then grounds the selected qubit and activatesa current source to drive one or more current pulses between bulksuperconductor 10 and the island 20 of the selected qubit. The currentpulses can have a period on the order of one or more picoseconds. Thevoltmeter, which can be a DC voltmeter, measures a time-average of thevoltage drop that the picosecond pulses cause. Thermal excitations inthe qubit can cause fluctuation in the measured potential difference.Thus, at low enough temperatures (less than 1 K), the effects of thermalfluctuations will be minimized, and the voltmeter can integrate thesignal infinitely long. The voltmeter generally requires detectionsensitivity on the order of microvolts, given the noise due to thehigh-resistance barrier.

As noted above, each qubit has a switch such as a SET between theassociated island 20 and ground. When the switch is closed, the qubit isgrounded and the wavefunction collapses, thus fixing the state of thequbit and allowing a current to flow through the qubit. The currentsource, which is in series to ground and the bulk superconductor,applies the current pulses through the grounded qubit to create apotential drop to which the electrode system on the respective qubitresponds. The measured potential drop indicates the magnetic momentstate of the selected qubit.

FIG. 5 illustrates an embodiment of control system 80 in a multi-qubitsystem 500. In the embodiment of FIG. 5, each qubit is read out in turn,but the evolution of the qubit system, e.g., the oscillation frequencyof each isolated qubit between its two states, will continue after thestart of the readout process. However, each qubit that further evolvesafter completion of the calculation will return to the desired stateafter some period of time t that can be determined for the qubit. Amethod for reading out the state of the qubit system, thus can ground afirst qubit, apply a current across the first qubit, measure thepotential drop across the width of the first qubit, and wait the time t(from the start of the readout on the first qubit) before repeating themethod for a next qubit.

FIG. 5 illustrates an embodiment of a control system for a multi-qubitsystem 500. Control system 80 includes readout logic, which is not shownbut is used to control the state of each of the devices in controlsystem 80, a single current source 84, grounding switches 40-1 to 40-Nfor respective qubits 1 to N, voltmeters 82-1 to 82-N for respectivequbits 1 to N, and electrode structures on the junctions of the qubits.In accordance with an aspect of the invention, one method for readout ofthe state of the multi-qubit system 500 grounds a first qubit by closingthe respective switch 40, and applies a current from the current source84 to the bulk of the multi-qubit system 500. Given that a single qubitis grounded, the current flows through just the grounded qubit,resulting in a voltage drop across the readout structure placed over thejunction. The respective voltmeter 82 can measure the voltage todetermine the state of the grounded qubit. The process can then berepeated for the remainder of the qubits in the system. In an embodimentof the readout method, only the states of specific qubits are readout,while the others continue to evolve.

FIG. 6 illustrates an embodiment of the control system for a qubitsystem 600 that includes a plurality of qubits and a control system 86for reading the states of the qubits. Control system 86 includes currentsource 84-1 through 84-N connected to islands 20-1 through 20-N,respectively in qubits 1 to N. Other than having a separate currentsource for each qubit aspects of control system 86 are as describedabove in regard to FIG. 5. However, a readout method for qubit system600 can include simultaneous readout of several or all of the qubits. Inan embodiment of the readout method, qubits in the multi-qubit system600 are grounded, and respective current sources 84-1 to 84-N drive acurrent through the respective qubits. Potential drops can be measuredin the readout structure on the respective qubits. Control system 86includes readout logic for controlling each of the control systemreadout components. In another embodiment of the readout method for thecontrol system of FIG. 6, the state of a qubit can be determinedindependently of the entire register using the method as describedabove.

FIG. 7 illustrates an embodiment of the invention, wherein controlsystem 86 has a single voltmeter 82 for all of the qubits. In thisembodiment, voltmeter 82 can measure the potential drop across theselected qubit, but each qubit must be read out individually. Theseparate readout operations for the individual qubits are timedaccording to the calculated time t for the qubits to return to thedesired state.

Although the invention has been described with reference to particularembodiments, the description is only an example of the invention'sapplication and should not be taken as a limitation. Various adaptationsand combinations of features of the embodiments disclosed are within thescope of the invention as defined by the following claims.

We claim:
 1. A method for reading out a state of a qubit, comprising:grounding the qubit; applying a current through the qubit; measuring avoltage across the current; and determining the state of the qubit fromthe voltage measured.
 2. The method of claim 1, wherein measuring thevoltage comprises measuring voltage between two separate electrodes thatoverlie opposite sides of the qubit.
 3. The method of claim 1, whereinapplying the current comprises applying current pulses through thequbit.
 4. The method of claim 3, wherein the current pulses have aperiod less than 10 picosecond.
 5. The method of claim 3, whereinmeasuring the voltage comprises using a DC voltmeter to determine a timeaverage of the voltage.
 6. The method of claim 1, wherein the qubitcomprises a first superconducting region and a second superconductingregion separated by a junction.
 7. The method of claim 6, whereinmeasuring the voltage comprises measuring voltage between two separateelectrodes, each of which overlies the junction.
 8. The method of claim6, wherein the current flows through the junction.
 9. The method ofclaim 1, wherein the qubit is a phase qubit.
 10. The method of claim 9,wherein the phase qubit is associated with a junction between a bulksuperconductor and a superconducting island, and applying the currentthrough the qubit comprises connecting a current source in series with abulk superconductor, the superconducting island, and ground.
 11. Themethod of claim 10, wherein the current source is a supercurrent source.12. The method of claim 10, wherein applying a current includes applyinga potential difference across the qubit with respect to ground.
 13. Themethod of claim 1, wherein the qubit is a permanent readoutsuperconducting qubit.